The invention relates to a system for assembly of tools within tool holders and, more particularly, apparatus and methods for accurately positioning a tool within a shrink-fit tool holder.
In the machining industries, it is well known to utilize cutting tools for performing various types of cutting operations on work pieces. Numerous types of cutting machines utilize tools which are positioned within tool holders. The tool holders are mounted on spindles or the like which, in turn, are associated with various types of machine tools.
One important aspect of machining processes relates to accuracy of work piece cutting and, more particularly, to accurate positioning of the cutting tool relative to the work piece. This activity is often referred to as high tolerance machining. Such accuracy is required in all three dimensions. To achieve the requisite accuracy, it is known to employ what is commonly referred to as “shrink-fit” clamping or holding of tools within tool holders. For example, Marquart, U.S. Pat. No. 5,992,860 issued Nov. 30, 1999, discloses a chucking device for mounting a tool on a machine tool. The machine tool includes a mounting section for mounting the chucking device. The machine tool also includes a thermally expandable shrink section. This shrink section includes a holding section which is intended to receive the tool in a shrink-fit relationship. An advantage of utilizing shrink-fit clamping of tools within tool holders is that such clamping provides a substantially concentric configuration. That is, the tool tends to be positioned (and also tends to remain) in a substantially centered configuration. In addition, shrink-fit clamping of tools will typically permit high speed rotation of the tools themselves, without problems such as “jittering” of the tools within the tool holders.
Shrink-fit clamping of tools is particularly well known in the CNC machine markets. However, although precision in positioning tools within shrink-fit tool holders can be readily achieved in two dimensions (i.e., accurate centering of the tool), conventional methods of clamping tools within tool holders in a shrink-fit configuration do not currently provide high accuracy in setting the axial position or “set length” of the tool.
For many types of conventional CNC equipment, high accuracy positioning of the tool within the tool holder is not of primary importance. For example, in a CNC machine employing a drilling head with a single drilling tool positioned within a shrink-fit tool holder, the tool position, relative to a table or the like holding the work piece to be drilled, can be essentially “calibrated.” That is, the CNC machine can be positioned so as to axially move the drilling tool and/or the table so that the terminal end of the drilling tool is initially positioned at what could be characterized as a “zero” or “base” point along a z-axis. This capability of relative positioning of the table and the drilling tool can compensate for lack of high tolerances in the positioning of the tool within the tool holder.
The foregoing description of relative positioning of a drilling tool and a work piece table may be sufficient for certain types of CNC or similar machine tool equipment. However, for other somewhat “nonconventional” or “specialized” CNC or similar machine tools, relative movement of a cutting tool and a work piece table may not be realizable. For example, machine tools in the automotive industry will often use a single machine tool head having multiple drilling or cutting tools, all possibly operating on a single work piece or the like. In this type of multiple tool configuration, it is not possible to adjust the zero or base points of the tools merely by relative repositioning of the tools and the work piece table. That is, as an example, the terminal ends of the cutting tools may be required to all be positioned within the same x-y plane. However, initial positioning of each of the tools within its respective tool holder may not permit the terminal ends of the tools to be positioned within a single plane.
Another issue which arises in the machining industries relates to growth/shrinkage of machine tool components in response to temperature changes. Such growth/shrinkage can readily effect accuracy in positioning of tools. This issue is recognized in Keehn, U.S. Pat. No. 5,421,683 issued Jun. 6, 1995. The Keehn patent discloses a gear shaving machine for cutting a series of work piece gears. The machine includes a computer-operated servomotor, servo-driver and computer for compensating for machine tool growth/shrinkage. The compensation facilitates uniformity in the depth of cut on successive work pieces. Although the Keehn patent deals with temperature compensation, it is not directed to any type of configuration for accurately positioning a tool within a shrink-fit tool holder. Instead, control is provided so as to regulate infeed distance. Also, the Keehn patent primarily relates to adjustments required as a result of thermal instability of the entirety of the machine tool.
Numerous other patent references are directed to concepts associated with machine tools and apparatus and methods for positioning tools within tool holders. For example, Yamaguchi, et al., U.S. Pat. No. 5,140,739 issued Aug. 25, 1992, is directed to a specific type of spindle unit and tool clamping article for machine tools. More specifically, a spindle head rotatably mounts a main spindle having an adapter which is shrink-fit on one end thereof. The adapter includes a sub-shaft portion onto which is positioned a shrink-fit tool holder. The tool holder mounts a conventional grindstone.
Bulen, U.S. Pat. No. 5,920,974 issued Jul. 13, 1999, discloses a reconfigurable gantry tool, with a reconfigurable tool system. The reconfigurable system includes a series of gantry tools which are coupled to one another so as to form a non-matrix assembly line. The illustrated embodiment is directed to machining of precision drill holes on work pieces involving the aircraft industry. Linear actuators and CNC systems are disclosed for precision drilling. Hira, U.S. Pat. No. 5,444,640 issued Aug. 22, 1995, generally discloses the concept of numerical control methods for correction of thermal displacement in machine tools.